The present invention relates generally to peak detector circuits, and more particularly, to circuits for detecting the peak amplitudes of extremely short-duration repetitive pulses.
The simplest form of peak detector is the conventional serial connection of a diode and capacitor to which an electrical signal is applied. The signal peak, minus the voltage drop of the diode, is stored in the capacitor and the output is taken thereacross. For large signals, for example, 50 volts or more, the half-volt diode drop is considered negligible; however, for the low-voltage signals prevelant in modern electronic equipment, even a half-volt diode drop is intolerable.
It is well known in the art to utilize a differential comparator in conjunction with a conventional peak detector circuit for generating signals corresponding to an incoming signal peak or a percentage thereof. It is also known to utilize a differential comparator having a feedback loop which includes a peak detector for producing a D.C. voltage level corresponding to the peak amplitude of an incoming analog signal of relatively low frequency. However, peak detection of short-duration repetitive pulses, particularly those having low repetition rates, presents a special problem in that not only must the peak amplitude be accurately detected but also a D.C. voltage level corresponding thereto must be provided. Generally, prior art pulse peak detectors have employed a first capacitor having a short time constant for charging quickly to the peak voltage, and a second capacitor having a fairly long time constant for holding the peak value between pulses.
In multi-function test and measurement instruments, the input comparator may be used in modes other than peak detection. One such mode may be a count mode wherein the comparator produces output pulses to be applied to counter circuitry in response to the comparison of an input signal with an adjustable reference level. To reduce noise effects on the comparator, and thereby minimize false count due to power supply noise spikes, an RC network having a rather long time constant is connected to the reference input of the comparator. In the peak detection mode, this RC network results in a long feedback response time, which may be, for example, 20 milliseconds or more.
Furthermore, it would be desirable in many applications to detect both positive and negative peaks wherever they occur in the amplitude range, whether above or below ground reference.